<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>正文居中</title>
    <style>
        .app
        {
            background-color: green;
            height: 300px;
        }
        .root-wrapper
        {
            background-color: brown;
            display: flex;
            justify-content: center;
        }
        .web-paper
        {
            background-color: wheat;
            width: 600px;
            height: 400px;
        }
    </style>
</head>
<body>
    <div class="app">
        <!-- 正文 -->
        <div class="root-wrapper">
            <div class="web-paper">
                <h1>正文</h1>
                <!-- 页脚 -->
                <h3>页脚</h3>
                <p>小字</p>
            </div>
        </div>
        
    </div>
</body>
</html>